Plant growth promoting microbes (PGPMs), such as plant growth-promoting rhizobacteria (PGPR), have gained worldwide importance and acceptance for agricultural benefits. PGPMs can affect plant growth by different direct and indirect mechanisms. Some examples of these mechanisms, which can be active simultaneously or sequentially at the same or different stages of plant growth, include (1) increased mineral nutrient solubilization and nitrogen fixation (i.e., making nutrients more available for the plant); (2) repression of soilborne pathogens (e.g., by the production of hydrogen cyanide, siderophores, antibiotics, and/or competition for nutrients); (3) improving plant stress tolerance to drought, flooding, salinity, and metal toxicity; and (4) production of phytohormones such as indole-3-acetic acid (IAA). Moreover, some PGPMs produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which hydrolyses 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in plants. By lowering ethylene concentration in seedlings and thus its inhibitory effect, these PGPMs stimulate the root length of seedlings. Some exemplary groups of PGPMs can be found among the phyla: Cyanobacteria, Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. There is a considerable amount of ongoing scientific research directed to understanding PGPMs, including the aspects of their adaptation, effects on plant physiology and growth, induced systemic resistance, biocontrol of plant pathogens, bio-fertilization, viability of co-inoculation, interactions with plant microorganisms, and mechanisms of root colonization.
By virtue of their rapid rhizosphere colonization and stimulation of plant growth and/or yield, there is currently considerable interest in exploiting PGPMs to improve crop production. In fact, the inoculation of cultivated plants with PGPMs is currently considered a promising agricultural approach. As environmental concerns increase, e.g., concerns about groundwater quality with excess fertilizer and pesticide exposure in foods, biological alternatives are promising and becoming necessary. Thus, developing biological treatments compatible with fertilizers and pesticides and/or even reducing the amount of these chemical compounds used could be a significant advancement in the agricultural industry.
However, there is a lack of efficient screening and selection procedures for obtaining microbial strains that have plant health/growth/yield promoting abilities. There is also a lack of efficient selection methods for obtaining combinations of microbial stains (or microbial consortia) that interact synergistically in the context of promoting plant health, growth and/or yield. The lack of such screening and/or selection procedures, unfortunately, slows down the study of plant-bacterial symbioses, and the deployment of new PGPMs in agriculture. Therefore, there is a continuing and pressing need for the identification of new PGPMs, PGPM synthetic consortia, and/or testing of their compatibility with existing commercially available crop management products.